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Radon Mitigation is any process used to reduce radon concentrations in the breathing zones of occupied buildings. The most common approaches are active soil depressurization (ASD) and mechanical ventilation (MV). Experience has shown that neither is applicable to all buildings with radon problems. A less common approach works efficiently by reducing air pressures within cavities of exterior and demising walls where radon emitting from building materials, most often concrete blocks, collects.

• Nearly all mechanical ventilation-based radon control systems are of fixed rate operation, and even if the indoor relative humidity in the interior of a building goes high, they will continue to inject moisture-laden air into this wet environment increasing the likelihood of mold growth. This is especially risky in hot, humid climates.
• Above Slab Air Pressure Differential Barrier technology ASAPDB requires that the interior pressure envelope, most often drywall, as well as all ductwork for air conditioning systems, be made as airtight as possible. A small blower, often no more than 15 cubic feet per minute may then extract the radon-laden air from these cavities and exhaust it to the out of doors. With well-sealed HVAC ducts, very small negative pressures, perhaps as little as 0.5 pascal, will prevent the entry of highly radon-laden wall cavity air from entering into the breathing zone. Such ASAPDB technology is often the best radon mitigation choice for high rise condominiums as it does not increase indoor humidity loads in hot humid climates, and it can also work well to prevent mold growth in exterior walls in heating climates.
• In hot, humid climates, heat recovery ventilators HRV as well as energy recovery ventilators ERV have a record of increasing indoor relative humidity and increased dehumidification demands on air conditioning systems. In some cases, it is not unlikely that serious mold problems have an origin in HRV and ERV operation in hot, humid climates. HRVs and ERVs have an excellent record in heating climates.
• A recently revealed technology is based in building science. It includes a variable rate mechanical ventilation system that prevents indoor relative humidity from rising above a preset level such as 50% which is currently suggested by the USEPA and others for the prevention of mold. It appears to be especially important for radon mitigation in hot, humid climates.

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